A long term evolved-advanced (Long Term Evolved-Advanced, LTE-A) system needs to support bandwidth being wider than that of an LTE system, so that multiple carriers may be aggregated. Carrier aggregation means that multiple component carriers (Component Carriers, CCs) are converged to a carrier with bandwidth being wider than that of the LTE system, to support a high-speed data rate.
In the prior art, when a user equipment (User Equipment, UE) uses multiple CCs, under certain scenes, a time advanced (Time Advanced, TA value) of each CC is different. For example, when a base station configures different CCs on different repeaters (Repeaters), or the CCs of different frequencies pass through different scattering paths, TA values of the different CCs are different. Further, for example, when the UE communicates with two non-collaborated access points by using two different CCs, the TA values of the two CCs are different.
Therefore, when the TA values of the multiple CCs used by the UE are different, if the base station needs to configure a new CC for the UE, or one of multiple CCs currently used by the UE needs to be replaced by newly configured CCs, how the UE can achieve uplink synchronization with the newly configured CCs becomes an urgent problem to be solved.